Charge-transporting polymers such as poly(vinyl-carbazole) (PVK) are promising materials for use as a photoconductive material for electrophotographic photoreceptors or as an organic electroluminescent element material, as described in Lecture Meeting of the Applied Physics Association, 37th., Preprints of Papers, 31p-K-12 (1990). In both of these applications, a charge-transporting polymer is used in the form of a charge transport layer. Well known materials for forming a charge transport layer include charge-transporting polymers such as PVK and low-molecular weight compound dispersions composed of a charge-transporting low-molecular weight compound dispersed in a polymer. In organic electroluminescent elements, a vapor-deposited low-molecular weight charge transport material is generally used. Of these charge transport materials, the low-molecular weight compound dispersions are mainly used especially in electrophotographic photoreceptors because they have an advantage that a variety of materials can be used so that highly functional devices are apt to be obtained.
Electrophotographic photoreceptors have come to be used also in high-speed copiers and printers with the recent trend toward performance increase in organic photoreceptors. However, the current performance of electrophotographic photoreceptors is insufficient for use in high-speed copiers and printers. With respect to organic photoreceptors, in particular, it is desired to further prolong the life thereof. One of the determinative factors of the life of an organic photoreceptor is the wear of the charge transport layer. The recent low-molecular weight compound dispersion type charge transport layers, which are mainly used at present, exhibit fully satisfactory performance with respect to electrical properties. However, such charge transport layers still have a drawback that since they are composed of a dispersion of a low-molecular weight compound in a polymer, they essentially have poor mechanical strength and hence poor resistance to wear. In the case of organic electroluminescent elements, there is a drawback that the low-molecular weight charge transport material melts due to the Joule's heat generated therein and undergoes crystallization, etc., and this tends to result in a morphological change of the film.
On the other hand, charge-transporting polymers have been intensively studied at present, because there is the possibility that the above-described drawbacks might be obviated with these polymers. For example, a polycarbonate obtained by polymerizing a specific dihydroxyarylamine with a bischloroformate is disclosed in U.S. Pat. No. 4,806,443, while a polycarbonate obtained by polymerizing a specific dihydroxyarylamine with phosgene is disclosed in U.S. Pat. No. 4,806,444. In U.S. Pat. No. 4,801,517 is disclosed a polycarbonate obtained by polymerizing a bishydroxyalkylarylamine with either a bischloroformate or phosgene. In U.S. Pat. Nos. 4,937,165 and 4,959,288 is disclosed a polycarbonate obtained by polymerizing a specific dihydroxyarylamine or bishydroxyalkylarylamine with a bischloroformate or a polyester obtained by polymerizing the specific amine with a bisacyl halide. Further, a polycarbonate or polyester obtained from an arylamine having a specific fluorene skeleton is disclosed in U.S. Pat. No. 5,034,296, and a polyurethane is disclosed in U.S. Pat. No. 4,983,482. Furthermore, polyesters having a backbone comprising units of a specific bisstyrylbisarylamine are disclosed in JP-B-59-28903, wherein the polyesters are used for sensitization in the form of an eutactic crystal complex formed with a pyrylium pigment salt. (The term "JP-B" as used herein means an "examined Japanese patent publication".) Moreover, polymers having a charge-transporting substituent such as a hydrazone or a triarylamine as pendant groups and photoreceptors using these polymers are proposed, for example, in JP-A-61-20953, JP-A-1-134456, JP-A-1-134457, JP-A-1-134462, JP-A-4-133065, and JP-A-4-133066. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) Of these polymers, the polymer having a tetraarylbenzidine skeleton is especially suitable for practical use because it has high mobility as reported in The Sixth International Congress on Advances in Non-impact Printing Technologies, 306 (1990).
Charge-transporting polymers are required to have various properties including solubility, mobility, and matching in oxidation potential. A generally employed technique for satisfying these requirements is to incorporate a substituent to control properties. Since the ionization potential of charge-transporting polymer is mostly determined by the charge-transporting monomer(s), it is important to use charge-transporting monomers whose ionization potentials are controllable. Monomers used as a starting material for the triarylamine polymers mentioned above are roughly classified into two groups, (1) dihydroxyarylamines and (2) bishydroxyalkylarylamines. However, the dihydroxyarylamines are susceptible to oxidation because of the possession of an aminophenol structure and are hence difficult to purify. In addition, although the dihydroxyarylamines are more instable especially when they have a para-hydroxy-substituted structure, it is difficult to control the ionization potential thereof by changing the positions of the substituents. A further problem of the dihydroxyarylamines is that since they have a structure in which the oxygen atoms of the substituents are directly bonded to the aromatic ring, they tend to have an inhomogeneous charge distribution due to the electron-withdrawing nature of the oxygen atoms and are hence apt to have reduced mobility. On the other hand, the bishydroxyalkylarylamines have an advantage that the influence of electron-withdrawal by the oxygen atoms is eliminated due to the alkylene groups, but have a drawback that monomer synthesis is difficult. That is, the reaction of either a diarylamine or a diarylbenzidine with 3-bromoiodobenzene tends to yield a reaction product in the form of a mixture because both the bromine and the iodine are reactive, resulting in a reduced yield. Another problem is that both the alkyllithium used for displacing bromine with lithium and ethylene oxide are highly dangerous and toxic and care should be taken in handling these.